The number of dealerships in the United States last year remained relatively flat, with pockets of increases. But with a recovering economy driving sales higher, the average number of new-vehicle sales per dealership surged, according to a study released [Monday] by Urban Science. That trend is expected to continue over the next three to four years, said John Frith, a vice president at Urban Science, a retail consulting company in Detroit. The 2013 Automotive Franchise Activity Report showed a slight dip in the number of U.S. dealerships. It projects the overall retail network will remain basically stable for a fourth straight year in 2014. As of Jan. 1, there were 17,838 dealerships, or rooftops, a 0.1 percent decrease from 17,851 a year earlier.